When signing up for Plus membership, you will receive a code that can be applied at Checkout. You'll only need to apply this code once, and as long as your a Marvel Unlimited Annual Plus member, you will automatically get 15% off your purchase at Checkout. Woot!
Below are images of where you can apply that code, and also a handy list of the different Unlimited Membership Plans. (Note: this promotion cannot be used on Subscriptions, Bundles, or sale items.)